I walk a mile in the morning, which sounds like a lot but it's only 15-20 minutes. I also try to walk another mile in the evenings and get to the gym 3-5 times a week. 6 times if I'm being good.
One of the best things I've discovered recently is walking backwards. It is SO good for the knees! There's a section on my daily walk that has a low fence, I put the fence on one side of me and have my husband on the other side of me and I walk backwards. It works and strengthens knee muscles. Sometimes I'll use the treadmill at the gym.
I don't often have back pain, but when I do I find that walking helps a lot.